They say all dialogue should further understanding of the character or further the story. It also needs to convey distinct character voice, be entertaining, and convey the impression that the writer is worthy of being hired. It’s a lot, and people often get lost trying to do everything at once.

Here’s my trick:

I like to write things that are boring and completely on the nose at first. That way it starts with clarity. You can always art it up later (and sometimes I remember to do that).

Identify the function of the line, what is it actually doing. Once you know that angle, it’s easier to find a stronger line that sells the same underlying idea.

Take this scene from THE ROOM:

ORIGINAL

Mark: How was work today?
Johnny: Oh, pretty good. We got a new client and the bank will make a lot of money.
Mark: What client?
Johnny: I cannot tell you; it's confidential.
Mark: Aw, come on. Why not?
Johnny: No, I can't. Anyway, how is your sex life?

It’s pretty basic, but a good actor would look at the lines and find the underlining meaning so they could emotionally ground a performance. We can do the same:

FUNCTION

MARK: <Generic opening line for verisimilitude. Invitation for further conversation>
JOHNNY: <Vague humblebrag about my important job.>
MARK: <Request for more information.>
JOHNNY: <Blasé refusal to go into more detail.>
MARK: <Whiny request for more information in a way that communicates neediness and a lack of understanding of how the world works.>
JOHNNY: <Flat refusal. Awkward segue into sex talk.>

That’s my subjective take, and probably not what the original writer meant. Still, it’s better to write from a strong point of view. Any choice is better than no choice, and you can always make a new choice in the rewrite.

REWRITE

MARK: You look happy. Like you just fucked Marilyn Monroe without a condom.
JOHNNY: Let’s just say I scored at work. Big money. I’ll make VP for sure.
MARK: Oh? Which one? Rumor is you guys have been chasing Nakamura Industries.
JOHNNY: I could tell you, but I’d have to kill you.
MARK: Okay, but it’d better be worth it.
JOHNNY: You’re pushing too hard. When was the last time you got laid?

A lot of times when people pitch me lines, they’re not great and I turn them down flat. But the more useful takeaway is to consider the ANGLE at which the line is pitched, the underlining meaning. Different angles yield different kinds of lines.

I’d call that better, but still not great. But it tells us a little bit more about the characters, advancing our understanding, making it easier to make the next draft even more specific.

If Johnny’s being casual about refusing detail, then we need the kind of thing casual people say: “I could tell you but then I’d have to kill you.”

Or he could be <evasive because he’s fronting about imaginary success> so “Uh, it’s good. Real good. Lotta boring details, but profitable.” Might do the trick.

Or it could be <Setting up labored segue into sex life because I really want to ask about yours> in which case the better line might be, “Ugh, I’m just glad it’s over. I’ve been so busy working that I haven’t been laid in a month.”

William Goldman once said that screenwriting was structure. Valid point, but it’s not only structure, it’s texture to, the the difference between “They have sex” and “As Bob runs his tongue over her neck, Alice grabs at the bed sheet with her toes.” It’s the little details, cool visuals, quotable dialogue, neat moments of behavior that bring it to life. Without it, even the most “properly” structured story will just lie there, inert.

Those specific moments, the little things that register vary from writer to writer, from audience to audience. A hundred writers will have a hundred takes on the same basic scene. You want to make sure every line in a scene advances your story and sells your basic competence as a writer. The easiest way to do that is by understanding what each line is actually doing in the scene.